1. Field of the Invention
The present invention relates to cleaning and coating of coiled tubing, and more particularly to such processes which use electro plasma.
2. Description of Related Art
In the oil and gas industries, coiled tubing refers to metal piping, normally 0.75″ to 4.25″ in diameter, used for interventions in oil and gas wells and sometimes as production tubing in depleted gas wells, which comes spooled on a large reel. Coiled tubing is often used to carry out operations similar to wirelining. The main benefits over wireline are the ability to pump chemicals through the coil and the ability to push it into the hole rather than relying on gravity. A coiled tubing operation is normally performed through the drilling derrick on the oil platform, which is used to support the surface equipment, although on platforms with no drilling facilities a self-supporting tower can be used instead. For coiled tubing operations on sub-sea wells a Mobile Offshore Drilling Unit (MODU) e.g. semi-submersible, drillship etc. has to be used to support all the surface equipment and personnel, whereas wireline can be carried out from a smaller and cheaper intervention vessel. Onshore, coiled tubing operations can be run using smaller service rigs, and for light operations a mobile self-contained coiled tubing rig can be used. Coil tubing can perform almost any operation for oil well operations if used correctly.
Coiled tubing can also be used for pumping and dispersing fluids to a specific location in the well, such as for cementing perforations, performing chemical washes, or fracturing formations. Coiled tubing umbilical technologies enable the deployment of complex pumps which require multiple fluid strings on coiled tubing. In many cases, the use of coiled tubing to deploy a complex pump can greatly reduce the cost of deployment by eliminating the number of units on site during the deployment.
One operation where coiled tubing is utilized is “Tracking” a well, a process which involves pumping large amounts of fluid, under high hydraulic pressure to fracture the oil or gas formation. Typically, hydrochloric acid (HCl) is one of the components of the frack fluid and the combination of acid and high temperature is highly corrosive to the system, one component of which is the coiled tubing. Internally and in most cases externally, coiled tubing is not protected against corrosion and is by its nature, highly susceptible to acid or hydrogen sulfide attack.
The electro-plasma process (EPP) disclosed herein and in U.S. Pat. No. 6,585,875, whose disclosures are expressly incorporated herein by reference, provide a means to protect the coiled tubing, both internally and externally by applying a single metal coating or layered coatings or alloy coatings. Additionally, because of the nature of the EPP, one type coating can be applied internally and another or different coating can be applied externally. Coiled tubing can be a small as from 0.50 inches in diameter up to 4 inches in diameter and roll formed from a single sheet of steel and continuously welded as forming occurs.
UK-A-1399710, U.S. Pat. No. 5,958,604, U.S. Pat. No. 5,981,084, U.S. Pat. No. 5,700,366 & U.S. Pat. No. 6,585,875B1. Electrical plasma processing [high voltage] operates in an electrical regime in which the current decreases or remains essentially the same as voltage is increased and are characterized by the formation of plasma at the onset of the unstable region and further characterized; “it should be clearly understood that the required bubble regime cannot be obtained with any arbitrary combination of variables, such as, gap, flow rate, electrolyte concentration, temperature and so forth”.
UK-A-1399710 teaches that the gas film must be continuous and the electrical regime which describes the current as decreasing or remaining constant as voltage is increased described the “unstable regime” characterized as the descending half of the first current curve.
WO-A-97/35051 describes an electrolytic process for cleaning and coating electrically conducting surfaces in which the anode comprises a metal for metal coating of the surface of the workpiece. In WO-A-97/35051 and 35052 an arc discharge or electro-plasma is formed on the surface of the workpiece and is established within the bubble layer. If the anode is constructed from a non-inert material, such as a non-refractory metal, then metal atoms are transferred from the anode to the cathode. Coatings achieved by this regime, such as in WO-A-99/15714, are a special form of electroplating because they occur at high voltage in the presence of an arc discharge and the plating is faster than normal electroplating.
U.S. Pat. No. 4,360,410 describes the use of foam for an improved electroplating process. This is a typical electroplating process where low voltage is utilized for ion transfer, without discharge or plasma generation. It operates in a different electrical regime with is typical conventional electrolytic processing. One of the teachings of this patent is that foam improves the uniformity of application.
WO-A-98-32892 describes a process which operates essentially in the manner described in WO-A-99/15714 but uses a conductive gas/vapor mixture as the conductive medium. This gas/vapor mixture is generated within a multi-chambered area by passing electrolyte through holes in the anode. The gas/vapor mixture is generated by heating an aqueous electrolyte within the chamber to the boiling point and the anode chamber may be heated either by primary electrical current or by independent electrical heaters.
WO-01/09410 A1-U.S. Pat. No. 6,585,875 describes a process similar to WO-A-98/32892 & WO-A-99/15714 and claims an improved process in which an electro plasma (arc-discharge) is utilized to clean and/or apply a metal coating to an electrically conductive substrate.
U.S. Pat. No. 6,585,875 teaches an improved process in which arc-discharge electro-plasma is employed to clean and/or apply a metal coating to an electrically conductive surface, in which the electrically conductive pathway is provided by a foaming electrolyte which fills the space between the anode and cathode and provides advantages with respect to lower power consumption, more uniform surface treatment and greater latitude in the size of the gap between the anode and cathode, thereby allowing for more diverse workpieces. Importantly, while the present invention employs the electro-plasma process generally described in U.S. Pat. No. 6,585,875, the present invention provides for substantial advances in the cleaning and coating processes as applied to coiled tubing as further described herein.